1. Field of the Invention
The present invention relates to a printing apparatus. A printing apparatus comprises a printhead in which first main nozzles having a first diameter and second main nozzles having a second diameter smaller than the first diameter are alternately arranged in the longitudinal direction on both sides of a common liquid chamber to supply a liquid. This printhead further comprises a plurality of liquid chambers having openings to the first and second main nozzles and communicating with the common liquid chamber. The present invention relates to a printing apparatus which drives and controls a printhead that prints on a printing medium by discharging liquids from first and second main nozzles, a printing apparatus control method, a printhead control circuit, and a printhead driving method.
2. Description of the Related Art
Along with the recent development of personal computers, the printer technology is also remarkably progressing. A printing apparatus is configured to print an image on a printing paper sheet on the basis of image information.
A printing scheme of a printing apparatus that has recently received a great deal of attention is an inkjet printing scheme. An inkjet printing apparatus discharges ink from a printhead to a printing paper sheet. This scheme allows high-speed printing of high-resolution images and is superior to other printing schemes in various points including the running cost and quietness.
The inkjet printing scheme is known to use an electrothermal transducer that generates thermal energy serving as ink droplet discharge energy. In this method, minute nozzles arranged on an inkjet printhead discharge minute ink droplets to print on a printing medium such as a paper sheet.
An inkjet printhead using electrothermal transducers includes a driving system to form ink droplets and a supply system to supply ink to the driving system. The electrothermal transducers are generally provided in a compression chamber. An electrical pulse serving as print data is applied to the electrothermal transducers to give thermal energy to the ink. An abrupt phase change of the ink, i.e., the pressure of bubbles generated upon vaporization at this time is used to discharge the ink.
The structure of a general inkjet printhead will be described here with reference to FIG. 2.
FIG. 2 is a perspective view showing the outer appearance of a general inkjet printhead.
Referring to FIG. 2, the inkjet printhead has nozzle arrays to discharge a plurality of color inks. A black (Bk) nozzle array 1 discharges black ink. A cyan (C) nozzle array 2 discharges cyan ink. A yellow (Y) nozzle array 3 discharges yellow ink. A magenta (M) nozzle array 4 discharges magenta ink.
The detailed structure of each nozzle array will be explained next with reference to FIG. 3.
FIG. 3 is a view showing the structure of nozzle arrays of an inkjet printhead.
As shown in FIG. 3, the mainstream of an inkjet printhead is a staggered nozzle arrangement. In the illustrated example, main nozzles for printing include 320 black (Bk) nozzles and 128 color (COLOR) nozzles (for each color) (FIG. 3 shows cyan, magenta, or yellow nozzles).
Each color nozzle array includes two arrays: an EVEN nozzle array of even-numbered nozzles on the left side and an ODD nozzle array of odd-numbered nozzles on the right side.
Each of the EVEN nozzle array and ODD nozzle array includes 160 nozzles for black and 64 nozzles for each color.
The positional relationship of nozzles will be described. A number of nozzles are arrayed at a predetermined pitch py in the y direction (sub-scanning direction) to form a nozzle array. Two nozzle arrays for the same color are arranged while being spaced apart in the x direction (main scanning direction) by a distance px corresponding to a predetermined number of pixels. The nozzles of the two arrays shift from each other by (py/2) in the y direction.
The main scanning direction is a direction to scan the inkjet printhead. The sub-scanning direction is perpendicular to the main scanning direction.
This structure ensures printing at a resolution (twice the resolution per array) by only adjusting the discharge timings of the two nozzle arrays.
In light of recent improvement of image quality, the size of ink droplets to be discharged is decreasing more and more to obtain high tonality. The color (Color) nozzles on the right side of FIG. 3 include nozzles (large nozzles) that discharge ink droplets in a conventional discharge amount and nozzles (small nozzles) that discharge ink droplets in almost ½ amount. FIG. 3 shows a structure including 128 large nozzles (●) of an ink discharge amount of 5 pl (one color) and 128 small nozzles (◯) of an ink discharge amount of 2 pl (one color).
The color (Color) nozzles will be described next.
The color (Color) nozzles shown in FIG. 3 have a two-array structure, as described above. Each color nozzle array includes an EVEN nozzle array of even-numbered nozzles on the left side and an ODD nozzle array of odd-numbered nozzles on the right side. The large nozzles (●) include 64 nozzles in the EVEN nozzle array of even-numbered nozzles and 64 nozzles in the ODD nozzle array of odd-numbered nozzles: a total of 128 nozzles. As for the positional relationship, the nozzles of each of the EVEN and ODD nozzle arrays are arranged at the predetermined pitch py in the y direction. The EVEN and ODD nozzles shift by (py/2). The EVEN and ODD nozzle arrays are arranged while being spaced apart in the x direction by the distance px corresponding to a predetermined number of pixels.
The small nozzles (◯) also include 64 nozzles in the EVEN nozzle array of even-numbered nozzles and 64 nozzles in the ODD nozzle array of odd-numbered nozzles: a total of 128 nozzles. The positional relationship is the same as that of the large nozzles except that the positions of the EVEN and ODD nozzle arrays are reverse of those of the large nozzles. That is, the ODD array is arranged on the left side (EVEN array of large nozzles), and the EVEN array is arranged on the right side (ODD array of large nozzles).
The EVEN and ODD nozzles shift by (py/2). The EVEN and ODD nozzle arrays are arranged while being spaced apart in the x direction by the distance px corresponding to a predetermined number of pixels.
The small nozzles (◯) and large nozzles (●) shift by (py/2) in each array. That is, each array includes large nozzles arranged at the pitch py and small nozzles arranged at the pitch py, which shift from each other by (py/2). This structure enables to add small nozzles to large nozzles at the same pitch without prolonging the nozzle array.
The schematic structure of a nozzle array will be described next with reference to FIGS. 4 and 5.
FIG. 4 is a view showing the schematic structure of a nozzle array of an inkjet printhead. FIG. 5 is a sectional view of the nozzle array taken along a line X in FIG. 4.
Especially FIG. 4 shows the schematic structure of a nozzle array of an inkjet printhead that discharges a predetermined color ink. Referring to FIG. 4, the inkjet printhead includes a plurality of main nozzles 5 to discharge ink, a plurality of ink chambers 6 with openings to the main nozzles 5, and a long common ink chamber 7 to supply the ink to the ink chambers 6.
The inkjet printhead of a color printer for multicolor printing has a plurality of nozzle arrays, and for example, four nozzle arrays shown in FIG. 4 in correspondence with four color inks, i.e., yellow, magenta, cyan, and black inks. In the above-described inkjet printhead, the main nozzles 5 are arranged at a pitch as small as possible to make the apparatus compact.
The inkjet printhead (to be abbreviated as a printhead hereinafter) handles a liquid. Hence, the printhead also includes a suction recovery mechanism to discharge a thick liquid from the printhead by using a cap, and a preliminary discharge (also called pre-discharge and executed independently of a print signal) mechanism to drive driving elements. Alternatively, a cleaning mechanism to clean the nozzle surface is applied to the inkjet printer.
The inkjet printer has operation sequences “cleaning”, “head refreshing”, and “wiping” to keep the main nozzle 5 of the printhead clean, as described above.
In the two former sequences, a negative pressure is applied to the cap that covers the main nozzles 5 to suck the ink in the common ink chamber 7, thereby eliminating clogging in the main nozzles 5. After that, preliminary discharge is performed. With wiping, thick ink sticking to the nozzle surface is removed.
Time preliminary discharge is executed even during printing at a predetermined time interval to prevent the ink in the main nozzles 5 unused for printing from thickening by time change and causing discharge errors in the next discharge sequence.
Independently of whether a plurality of color nozzle arrays are provided integrally or separately, liquids of different colors or different characteristics may mix between the printheads of respective colors. Various means for solving this problem are known.
Japanese Patent Laid-Open No. 8-295033 discloses a technique of providing dummy nozzles between adjacent printheads to prevent color mixing between them. More specifically, inks from adjacent printheads are guided to the dummy nozzles. The dummy nozzles discharge the mixed ink, thereby removing the mixed ink. The main nozzles discharge ink to print on a printing medium. The dummy nozzles do not discharge ink to print on a printing medium.
In Japanese Patent Laid-Open No. 2001-129997, some of the main nozzles 5 serve as dummy nozzles 8 along the array direction of the main nozzles 5, as shown in FIG. 6. A dummy ink chamber 9 of each dummy nozzle 8 connects to the common ink chamber 7 of the array of main nozzles 5. In the printhead recovery process, the dummy nozzles 8 can also preliminarily discharge ink that is staying in the common ink chamber 7.
Consequently, the dummy nozzles 8 discharge, together with the liquid, bubbles existing at the two ends of the common ink chamber 7 so that the mixed ink can immediately be discharged. This especially promotes liquid flow between the dummy nozzles 8 and the longitudinal ends of the long common ink chamber 7 to which the ink is supplied. Hence, the dummy nozzle 8 can smoothly and reliably discharge, from the printhead, thick ink that tends to stay at the longitudinal ends of the common ink chamber 7.
In printhead suction recovery by “cleaning” or “head refreshing” as described above, one cap sucks all nozzle arrays (black, cyan, magenta, and yellow) simultaneously. Alternatively, one cap sucks the black nozzle array or the color nozzle arrays (cyan, magenta, and yellow) simultaneously. For this reason, all inks mix in the cap.
The mixed ink in the cap may stick to the nozzle surface of the printhead and may be sucked by the negative pressure in the ink tank after the suction operation stops.
If printing is done in this state, the nozzles discharge inks of undesired colors, resulting in a large degradation in the printed image quality. To prevent this, the nozzles preliminarily discharge the mixed ink sucked in the printhead after suction recovery.
The above-described common ink chamber 7 shown in FIG. 6 is long and therefore readily stores ink at the longitudinal ends. The main nozzles 5 to be used for printing execute preliminary discharge. Then, the dummy nozzles 8 execute preliminary discharge after a predetermined time.
This allows to discharge, from the printhead, the thick ink that is staying at the longitudinal ends. Even in time preliminary discharge executed during printing, the main nozzles 5 used for printing and the dummy nozzles 8 execute preliminary discharge.
The preliminary discharge of the main nozzles 5 and dummy nozzles 8 will be described with reference to FIGS. 7A and 7B.
FIGS. 7A and 7B are views schematically showing preliminary discharge.
FIG. 7A particularly shows preliminary discharge from the main nozzles 5, and FIG. 7B shows preliminary discharge from the dummy nozzles 8.
When the main nozzles 5 execute preliminary discharge, as shown in FIG. 7A, ink is smoothly discharged from the common ink chamber 7 toward the main nozzle outlets but easily stays at the two ends of the common ink chamber 7. These portions will be referred to as stagnation portions 10 hereinafter.
The ink staying at the stagnation portions 10 readily stick. To prevent this, the dummy nozzles 8 execute discharge after discharge from the main nozzles 5, as shown in FIG. 7B, thereby discharging the ink staying at the stagnation portions 10.
Another reason why the main nozzles and dummy nozzles separately execute preliminary discharge will be described below.
A heater board with electrothermal transducers of an inkjet printhead has a limited size from the viewpoint of cost reduction. If it is impossible to arrange, within the size, DATA lines and HeatEnable lines like those for main nozzles, dummy nozzles must share the signal input DATA lines and HeatEnable lines of the block of main nozzles.
If the main nozzles and dummy nozzles of an inkjet printhead with such a wiring structure simultaneously execute preliminary discharge, the number of simultaneously discharging nozzles in a common block becomes more than expected, and the voltage drop increases. This may make preliminary discharge of the main nozzles in the block common to the dummy nozzles insufficient and cause color mixing due to insufficient ink discharge, resulting in an adverse effect on the image quality.
The number of shots of discharge in the preliminary discharge process is generally equal between the main nozzles 5 and the dummy nozzles 8. To execute preliminary discharge of the main nozzles 5 to be used for printing, the dummy nozzles 8 execute preliminary discharge of the same number of shots. That is, the preliminary discharge process of the main nozzles 5 and that of the dummy nozzles 8 are executed separately. This preliminary discharge process therefore requires a double time.
Even in preliminary discharge of small nozzles added by recent improvement of the image quality and resolution, a heater board with electrothermal transducers of an inkjet printhead has a limited size from the viewpoint of cost reduction, as described above. Hence, large nozzles and small nozzles share the signal input DATA lines and HeatEnable lines of each block to arrange DATA lines and HeatEnable lines dedicated to the large nozzles and small nozzles within the size.
It is impossible to make the large nozzles and small nozzles of an inkjet printhead with such a wiring structure simultaneously execute preliminary discharge. The small nozzles also execute the preliminary discharge process, like the conventional large nozzles.
Since the preliminary discharge of the large nozzles and that of the small nozzles are executed separately, the preliminary discharge process requires a longer time.